Demonstrations are planned in the US, Canada and Australia over the next three days in support of Manning, an army intelligence analyst who is being held at a military prison in Virginia.

The leaks include a cache of documents – published in the Guardian as the Afghan war logs – drawn from 76,000 pages of military records made public by WikiLeaks to challenge the US government's assertion that the fight against the Taliban is being won.

WikiLeaks says it will release thousands more pages of similar documents soon.

Manning was arrested in May after apparently boasting in instant messages and emails to a high-profile former hacker, Adrian Lamo, that he passed on the video and documents. Lamo said he turned in the soldier out of concern for national security.

Ellsberg, who leaked the Pentagon papers to the New York Times that laid bare the extent of US government duplicity in its claims to be winning the Vietnam war, said Manning was defending the constitution in revealing the truth about the conflicts in Iraq and Afghanistan.

"Soldiers' sworn oath is to defend and support the constitution. Bradley Manning has been defending and supporting our constitution," he said.

Moore plans to broadcast the main protest event live on his website. He said the US military was being hypocritical in its attempts to discredit Manning and accuse WikiLeaks by asserting that making the secret documents public endangered the lives of Afghans collaborating with coalition forces.

"To suggest that lives were put in danger by the release of the WikiLeaks documents is the most cynical of statements," Moore said.

"Lives were put in danger the night we invaded the sovereign nation of Iraq, an act that had nothing to do with what the Bradley Mannings of this country signed up for: to defend our people from attack. It was a war based on a complete lie and lives were not only put in danger, hundreds of thousands of them were exterminated.

"For those who organised this massacre to point a finger at Bradley Manning is the ultimate example of Orwellian hypocrisy."

Among others who have thrown their weight behind the campaign are a retired army colonel, Ann Wright, and a retired CIA analyst, Ray McGovern, who is described as having regularly prepared daily security briefings for former presidents Ronald Reagan and George HW Bush.

The US protests are being held in 18 cities. The primary event, in California, is organised by Courage to Refuse, which describes itself as a coalition of soldiers and civilians who encourage resistance by members of the forces to the wars in Iraq and Afghanistan.

The US defence department has established a team of intelligence analysts, FBI agents and others to combat WikiLeaks' threat to release tens of thousands more pages of documents.

The Daily Beast has reported that the operation, known formally as the Information Review Task Force, is being run by Brigadier General Robert Carr of the Defence Intelligence Agency, the Pentagon's equivalent of the CIA.

Carr's principal assignment is to attempt to establish which documents WikiLeaks has and to limit the damage in Afghanistan as well as diplomatically.